In a digital circuit, high frequency noises generated incidentally to switching operations of a semiconductor element cause electromagnetic interferences. This high frequency noise mainly includes high-order harmonics of the fundamental clock frequency. For example, a part of the high frequency noises generated in an element in an LSI (Large Scale Integration) that makes a switching operation (hereinafter referred to as a switching element) propagates through the power supply wiring in the LSI and further leaks through the package of interest into the power supply wirings etc. of the printed-circuit board having the LSI mounted on.
The high frequency noises, which propagate through the power supply wirings, couple with the package and signal wirings on the printed-circuit board, etc. in the LSI through electromagnetic induction in the propagation paths. This electromagnetic coupling causes the high frequency noises to superimpose on the signals carried through signal wirings to yield distortions of the signal voltages. Furthermore, in the case where the surge impedance of the power supply wiring viewed from the switching element is high, the creation of the high frequency noise brings about the creation of an electromagnetic wave, which is emitted from the signal cable and devices.
It is advantageous to arrange decoupling circuits adaptive to the frequencies of the generated high frequency noises in most effective positions in order to relieve the above problems.
In the conventional decoupling circuit as described, for example, in JP H10-270643, a capacitor equivalent to a lumped constant has been arranged between a power supply wiring and a ground potential wiring, because the sizes of the elements that constitute the circuit, such as a transistor, a resistor, a capacitor, etc., are small as compared with the signal wavelength corresponding to the operation frequency of the subject circuit.
Concerning other conventional decoupling circuits, JP 2001-168223 describes a technology of increasing the decoupling capacitance between ground and power supply rings; and JP H06-216309 describes a technology of providing a decoupling capacitor on a lead frame of the semiconductor device. For reference, the above technologies relate to decoupling capacitors inserted between a power supply wiring and a ground potential wiring provided on the same plane and fundamentally differ in the construction from the decoupling circuits of the present invention described later in that a power supply wiring and a ground potential wiring are formed in different layers.
In the circuitry in which a capacitor is employed as a decoupling circuit, it is necessary to take into account the inductance component included in series with the connection terminal for the noises in a high frequency range. In other words, a capacitor includes both capacitance and inductance components representing a capacitive characteristic at the frequency lower than the series resonance frequency of the capacitance and inductance and an inductive characteristic at the frequency higher than the series resonance frequency. Accordingly, when using a capacitor as a decoupling circuit, the decoupling circuit becomes more inductive as the frequency becomes higher, resulting in degradation of the decoupling performance.
As a countermeasure to address such an issue, there is a method in which many capacitors are arranged in a distributed configuration near the package, or in the printed-circuit board in the LSI. Even if this method is employed, however, the inductance of the terminals and transmission lines for connecting the capacitors and power supply wiring has an unnegligible magnitude. For this reason, it has been difficult to operate the capacitors as a decoupling circuit for a frequency range of several hundreds MHz or higher.
The operations of recent digital circuits have been sped up to have the operation frequencies as high as several GHz. Consequently, it is imperative that a decoupling circuit has an impedance kept at a low value in the frequency range higher than hundreds MHz, or preferably several tens GHz, in order to suppress an electromagnetic interference and improve a signal quality. For this end, it is necessary to develop a circuit element or an element structure differing from the conventional capacitor in order to be capable of retaining low impedance in a high frequency range.
It is an object of the present invention to provide a semiconductor device and a semiconductor circuit provided with a decoupling circuit capable of retaining a low impedance up to the frequency higher than several hundreds MHz, and preferably several tens GHz.